Observation of $\psi (3686)\to n\overline{n}$ and improved measurement of $\psi (3686)\to p\overline{p}$

We observe the decay $\psi (3686)\to n\overline{n}$ for the first time and measure $\psi (3686)\to p\overline{p}$ with improved accuracy by using $1.07\times {10}^8$ $\psi (3686)$ events collected with the BESIII detector. The measured branching fractions are $\mathcal{B}(\psi (3686)\to n\overline{n})=(3.06\pm 0.06\pm 0.14)\times {10}^{-4}$ and $\mathcal{B}(\psi (3686)\to p\overline{p})=(3.05\pm 0.02\pm 0.12)\times {10}^{-4}$. Here, the first uncertainties are statistical, and the second ones are systematic. With the hypothesis that the polar angular distributions of the neutron and proton in the center-of-mass system obey $1+\alpha {\cos }^2\theta$, we determine the $\alpha$ parameters to be ${\alpha }_{n\overline{n}}=0.68\pm 0.12\pm 0.11$ and ${\alpha }_{p\overline{p}}=1.03\pm 0.06\pm 0.03$ for $\psi (3686)\to n\overline{n}$ and $\psi (3686)\to p\overline{p}$, respectively.